An electric motor is a machine that converts electrical energy into mechanical energy. Electric motors include DC motors and AC motors, with AC motors increasingly being used.
AC motor is driven by 3-phase alternating current (AC) provided by an electric motor controller coupled to a 3-phase inverter. The AC motor includes an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field. The interaction of the rotor field and the stator field causes rotation of the rotor which can be used to perform work.
AC motors include induction or asynchronous motors which rely on a small difference in speed between the rotating magnetic field and the rotor to induce rotor current. A second AC motor type is the synchronous motor, which does not rely on induction and as a result can rotate at the supply frequency, or a sub-multiple of the supply frequency. The magnetic field on the rotor is either generated by a current delivered through slip rings or by a permanent magnet.
Stator resistance (Rs) is known to change with temperature, and stator winding temperature is known to increase as the average current through the stator winding increases. Thus, in a variable speed motor, when the drive voltage and current coupled into by the 3-phase inverter is increased or decreased, Rs is subject to change which in turn results in an unexpected demand in stator voltage. In order to achieve accurate motor control, accurate determination of Rs is needed. One method to determine Rs is by injecting DC current test signals into the stator windings and observing resulting variations in the phase currents and voltages of the motor.